1. Field
The present invention relates generally to control rod assemblies for nuclear reactors and, more particularly, is concerned with a flexible joint between a drive rod assembly and a control rod.
2. Related Art
In a typical nuclear reactor, such as a pressurized water reactor, the reactor core includes a large number of fuel assemblies, each of which is composed of top and bottom nozzles with a plurality of elongated, transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. In addition, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission, and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during operation, and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of neutron absorber control rods within the core. The level of neutron flux, and thus the heat output of the core, is normally regulated by the movement of the control rods into and out of the guide thimbles.
One common arrangement utilizing control rods in association with the fuel assembly can be seen in U.S. Pat. No. 4,326,919 to Hill. This patent shows a control rod spider assembly which includes a plurality of control rods and a spider structure supporting the control rods at their upper ends. The spider structure, in turn, is connected to a control rod drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of a control rod used in such an arrangement is in the form of an elongated metallic cladding tube having a neutron absorbing material disposed within the tube and end plugs at opposite ends thereof for sealing the absorber material within the tube.
The spider structure typically includes a plurality of radially extending vanes supported on and circumferentially spaced about a central hub. The vanes are flat metal plates positioned on edge and connected at their inner most ends to the central hub. Cylindrically shaped control rod connecting fingers are mounted to and are supported by the vanes with some of the vanes having only a single connecting fingers and other vanes having a spaced pair of connecting fingers associated therewith.
Typically, the upper end plug of each control rod has a threaded outer end which is received within a bore in the lower portion of a connecting finger of the spider structure and threadable into a tapped hole formed in the finger at the inner end of the bore. The control rod end plug is then secured or locked within the bore of the connecting finger by a key or pin inserted into the side of the finger and the end plug and the pin is then welded therein, as more particularly described in U.S. Pat. No. 4,855,100.
The current rod cluster control assembly control rods incorporate a reduced diameter section at the connection with the finger, which forms a flexure joint. The joint allows the control rod to deflect laterally to compensate for small misalignment between the control rod and the fuel assembly guide thimble or other reactor components, to minimize interference. This helps to prevent hang-up and reduces wear between the components. It also prevents bending loads induced by lateral deflection of the control rods from overloading the screw threads which attach the control rod upper end plug to the connecting finger. However, there are several limitations to the current flexure joint design. For example, lateral deflection with the current design must be limited by the spider finger instead of by the interfacing components to prevent overload of the joint. In addition, the spider finger hole diameter must be large enough to clear a shoulder on the control rod upper end plug, adjacent to the screw threads, that allows for preloading of the threaded joint and a reaction to lateral bending loads. The required diameter of the finger hole means that the wall thickness of the interfacing end of the finger is very small, which adversely impacts the spider assembly manufacturing process. Further, to ensure that the control rod is locked into position within the finger and cannot unscrew, the current rods use the small pin welded into a tightly toleranced hole drilled after the rods are installed into the spider finger. If the hole is drilled too deeply then the pin does not engage the spider finger and the rod can rotate and become disengaged from the assembly. In the past, this has resulted in a few dropped rods. Further, because the hole must be drilled after the rods are installed into the spider assembly, this operation slows the manufacturing process.
Accordingly, a new control rod/spider interface is desired that provides increased lateral deflection that is only limited by the interfacing components, e.g., a control rod guide thimble or other core internal components, instead of the spider finger.
Additionally, such an interface is desired that has the threaded joint preloaded from the bottom of the finger that enables fingers with thicker walls to improve the efficiency of the manufacturing process.
Additionally, such an interface is desired that has a weld at the bottom of the spider finger to lock the control rod in position, without having to drill holes for a pin.